Thermodynamics has established the interrelationship between various forms of energy, including heat and work. Moreover, thermodynamics has quantified this interrelationship, showing, for example, that in chemical and physiological processes the difference between the energy of the products and the energy of the reactants is equal to the heat gained or lost by the system. In an “exothermic” process, this difference is negative, so that the process releases heat to the environment. Conversely, in an “endothermic” process, this difference is positive, so that the process absorbs heat from the environment. Thus, “calorimetry,” or the measurement of heat production and/or heat transfer, can be used to determine if a chemical or physiological process is exothermic or endothermic and to estimate the energy produced or consumed.
The measurement of heat production and/or heat transfer in chemical and physiological processes can be quite complicated. Standardly, such measurements are made using a device known as a “bomb calorimeter.” This device typically includes a sturdy steel container with a tight lid, immersed in a water bath and provided with electrical leads to detonate a reaction of interest inside the calorimeter. The heat evolved in the reaction is determined by measuring the increase in temperature of the water bath.
Unfortunately, bomb calorimeters are inadequate for the measurement of heat production and/or heat transfer in many areas of chemistry and physiology. For example, the study of processes involving uncommon and/or expensive components may require analysis of samples too small for bomb calorimetry. Similarly, the high-throughput screening of pharmaceutical drug candidate libraries for drug activity may require analysis of too many samples for bomb calorimetry.
The analysis of small samples is especially problematic due to their small heat capacities and large surface-to-volume ratios. Many chemical and physiological processes lead to very small changes in temperature (<0.05° C.), making their analysis susceptible to environmental contamination. In particular, whenever there is a temperature difference between a sample and the environment, heat can be exchanged between the sample and the environment, for example, by conduction, convection, and/or radiation, among others. Such heat exchange may quickly alter the temperature of a small sample and thereby obscure any temperature change associated with a reaction. Moreover, fluid samples such as those typically used in studies of chemical and physiological processes may initiate secondary reactions with the environment, such as evaporation. Evaporation, by definition, is an exchange of energy (moisture is added to the air, while chemical volume is reduced). This process takes place on the surface of the sample, where the sample is exposed to the environment, and so may be especially problematic for small samples due to their relatively large surface-to-volume ratios. Evaporation not only removes energy from the sample, contaminating the measurement, but also may increase measurement noise due to surface instability as the fluid phase changes to a gas phase.